Iron(III) Chloride

Iron(III) chloride (ferric chloride, FeCl₃; CAS 7705-08-0) is the iron source for Prussian-blue toning of silver gelatin prints — the chemistry analogous to cyanotype but applied to an existing silver image rather than to raw sensitized paper.^[1] In a tone bath combining ferric chloride with potassium ferricyanide and a mild acid, the silver image is progressively converted to ferric ferrocyanide (Prussian blue), producing cool blue tones that intensify with extended treatment. Widely used in landscape and architectural print work for the distinctive "moonlight" aesthetic.

Photographic uses

• Blue toner (iron): The classical iron + ferricyanide formula. FeCl₃ + potassium ferricyanide + oxalic acid (or citric acid) in water, applied to a well-fixed silver print. Tonal shift from warm-black to intense blue progresses over 5–30 minutes depending on dilution.
• Bleach-back for Van Dyke / Kallitype: Dilute ferric chloride removes excess iron staining from over-developed alt-process prints, restoring paper whites.^[2]
• Photogravure etching: Concentrated ferric chloride is the classical mordant for etching copper plates in photogravure workflows.
• Iron intensifier: Some historical intensifier formulas use ferric chloride as the metal salt replacing the existing silver with an iron-silver complex for higher printing density.

Practical notes

Supplied as dark-yellow-to-brown crystalline flakes (the hexahydrate, FeCl₃·6H₂O — the common commercial form) or as a dark-brown aqueous solution (typically 40% w/w). Extremely hygroscopic and deliquescent: fresh flakes become slurry on exposure to humid air within hours. Store in tightly sealed containers with desiccant.

Solutions are strongly acidic (pH < 1 at working concentration). Dilute carefully with copious water; the diluted working strength is much more manageable than the concentrate.

Staining: ferric chloride stains skin, clothing, and porous surfaces yellow-brown to orange. Metal counter surfaces develop rust pitting from direct contact. Use in dedicated workspace.

Related compounds

Ferric ammonium citrate is the gentler iron source used in sensitizer chemistry (cyanotype, Pt/Pd). Ferrous sulfate is the iron(II) counterpart — different oxidation state, used as a developer rather than a toner.